<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            border: 1px solid red;
        }

        .Site {
            display: flex;
            min-height: 100vh;
            flex-direction: column;
        }

        header,
        footer {
            flex: 1;
        }

        .HolyGrail-body {
            display: flex;
            flex: 1;
        }

        .HolyGrail-content {
            flex: 1;
        }

        .HolyGrail-nav,
        .HolyGrail-ads {
            /* 两个边栏的宽度设为12em */
            flex: 0 0 12em;
        }

        .HolyGrail-nav {
            /* 导航放到最左边 */
            order: -1;
        }
    </style>
</head>

<body class="Site">
    <header>...</header>
    <div class="HolyGrail-body">
        <main class="HolyGrail-content">...</main>
        <nav class="HolyGrail-nav">
            是的房间可拉倒设计费看来是jdklsjflksda
            是的房间可拉倒设计费看来是jdklsjflksda
            是的房间可拉倒设计费看来是jdklsjflksda
            是的房间可拉倒设计费看来是jdklsjflksda

            是的房间可拉倒设计费看来是jdklsjflksda
        </nav>
        <aside class="HolyGrail-ads">...</aside>
    </div>
    <footer>...</footer>
</body>

</html>